Antennas are widely used for remote sensing, communications, and for industrial/therapeutic purposes. An antenna is simply a transducer between guided and unguided electromagnetic fields. An antenna, seen as a transducer, includes a “feed” port and a “radiating aperture” unguided or free-space radiation port. The feed port is so termed for historical reasons; early receiving antennas were simply pieces of electrically conductive wire which received little attention, while transmitting antennas received sophisticated attention because of the great effect that they could have on the high-power transmitters of the age. Thus, antennas were originally viewed as being transmitting devices defining one or more feed points. Only later was it discovered that antennas have the same radiation patterns and other characteristics in both the transmitting and receiving modes of operation. The feed port is ordinarily coupled to a “transmission line,” which is simply an electrical conducting arrangement having a defined (or at least controlled) surge or characteristic impedance. The electromagnetic energy flowing in the transmission line is guided by the line, and the radiation at the free-space port is in directions controlled by the “electrical field distribution” at the radiating aperture of the antenna. When the antenna operates in a receiving mode, free-space or unguided energy impinging thereon is transduced to become guided energy in the transmission line, and in the transmitting mode, guided energy applied to the feed port from the transmission line is radiated as unguided radiation (subject to certain limitations).
The field distribution characteristics of the radiating aperture of an antenna determine the “far-field” radiation pattern. One of the salient generalizations which can be made about antennas is that the radiating beam width is inversely related to the dimensions of the radiating aperture. That is, a highly directive antenna or radiation beam (a beam subtending a small sector of space) requires a large radiating aperture in terms of wavelength, and conversely a small radiating aperture results in a low-directivity or broad radiation beam. There are two popular ways to achieve a large radiating aperture in order to form directive antenna beams, namely (a) reflectors and (b) arrays.
An antenna array is an array including a plurality of antennas. When antennas are arrayed and properly phased, the overall radiation pattern is determined as the result of an “array factor” which multiplies the radiation pattern of the underlying antenna element of the array. Array antennas are of two general types, namely line (one-dimensional) arrays and surface (two-dimensional) arrays. The salient difference between these two is that the line array produces an array factor which multiplies the pattern of the underlying array only in the direction of the array, while a surface array produces a useful array factor in two mutually orthogonal directions. Thus, when a three-dimensional “pencil” beam is desired, it is likely that a two-dimensional surface array will be required. A plurality of line arrays can be juxtapositioned and fed so as to form a surface array, and a surface array can be viewed as being a plurality of interconnected line arrays.
As mentioned, it is necessary to feed the elements of an array antenna with signals of controlled phase in order to achieve the desired radiation pattern. The distribution of signals from a common feed point to the individual antenna elements of the array is often accomplished by a beamformer, which divides the available signal among the antenna elements, and which may include a phase shifter associated with each antenna element, or at least with subgroups of antenna element. The phase shifters are controlled in well-known manner in order to achieve the desired antenna beam direction. Each antenna element (or subgroup of antenna elements) of an array antenna may be associated with controllable attenuators and amplifiers as well as with phase shifters. In order to route the signals between and among the antenna elements, their amplifiers, phase shifters, and attenuators, if any, and possibly other elements, the array antenna will also include transmission lines. These transmission lines may take the form of hollow conductive waveguides, coaxial transmission lines, andor any one of various forms of “printed-circuit” transmission lines, such as finline, stripline or microstrip, all known in the art. Each transmission line must maintain its proper impedance to prevent the introduction of unwanted phase shifts andor attenuation, and remain electrically connected to its signal source and load.
Considering the complexity of array antennas, and all the potential problems which can arise due to degradation or failure of one or more of the amplifiers, phase shifters, attenuators, and transmission lines, it may be desirable to provide for some means for calibrating the antenna array in order to allow monitoring of its condition. One way to calibrate an array antenna is to compare it with a standard antenna, such as a horn antenna. That is, a source is coupled to the array and then to the horn, and the radiated power or energy at a substantial distance (the “far field”) in a particular direction is determined for each. The difference between the two represents the “gain” difference. As mentioned, this technique is quite suitable to a laboratory, but may not be easy to accomplish where the array is installed or located.
Another way to calibrate an antenna is to mount a test or calibration antenna near the antenna to be tested. Such calibrations are often known as “near-field” calibrations, and have the advantage of improved signal-to-noise ratio over the far-field technique. Signals are transmitted between the antenna being tested and the test/calibration antenna. Such a technique is described in U.S. Pat. No. 6,084,545, issued Jul. 4, 2000 in the name of Lier et al. In this patent, a calibration antenna or probe is placed in front of the array antenna to be tested, and the test signals are transmitted from the probe to the antenna being tested in the receive mode or from the antenna being tested to the probe in the transmit mode.
Another calibration method is described in U.S. Pat. No. 6,356,233, issued Mar. 12, 2002 in the name of Miller et al. This arrangement deems certain antenna elements of the antenna under test to be “kernel” elements, and uses mutual coupling between the kernel elements and the remainder of the antenna elements of the array to determine characteristics of the antenna. A system of switches and directional couplers routes test signals through the various kernel elements and their mutually coupled array elements.
Improved array antenna calibration arrangements are desired.